Miguel A. Ríos

An ab initio study of the conformation and intramolecular proton transfer in the enol form of acetylacetone

Abstract A detailed ab initio study of the enol form of acetylacetone is reported including both an exhaustive conformational analysis with total optimization, and an estimation of the variation of potential energy during interconversion of the two equivalent Cs symmetrical structures of lowest energy by intramolecular proton transfer. That the calculated frequency of interconversion, 1.8 × 1011 s−1, is lower than for malonaldehyde, the simplest β-diketone, appears to be largely due to coupling between methyl rotation and the proton-transfer process.

Ab initio study of interactions in methylamine clusters. The significance of cooperative effects

Methylamine clusters consisting of up to four molecules were studied by employing Hartree–Fock, density functional theory, and Moller–Plesset calculations with the 6-31+G* basis set. The dimer was found to exhibit two minima with similar interaction energies (−13 kJ/mol) and involving a hydrogen bond. The dipole moment for the dimer differs by up to 20% from the vector addition of the dipole moments for the individual molecules by effect of the interaction; also, the N–H bond distance in the group involved in the hydrogen bond is lengthened by up to 0.006 A as a result. The minima identified for the trimer and tetramer possess cyclic structures that differ in the position of the methyl groups with respect to the plane containing the hydrogen bonds. The contribution of nonadditivity to the interaction in these structures is quite significant (12%–18% of the overall interaction energy). N–H distances in the donor molecule are longer than in the dimer and increase with increasing cluster size. Likewise, the ...

Ab initio study of M(CH3CN)n clusters (M=Li+, Na+, Mg2+) in the gas phase

Abstract Calculations for clusters consisting of an Li + , Na + or Mg 2+ ion and up to six acetonitrile molecules were carried out using the HF, DFT/B3LYP and MP2 methods with the 6-31+G * basis set. The three methods led to the same minima, which correspond to highly symmetric structures, where the dipole moment of acetonitrile points at the cation in order to facilitate a charge–dipole interaction. Intermolecular distances follow the sequence Na + >Mg 2+ >Li + with the three methods used; on the other hand, interaction energies decrease in the sequence Mg 2+ >Li + >Na + . Intermolecular distances increase gradually with increasing cluster size. By contrast, the energy change resulting from incorporation of an additional molecule into a given cluster decreases with increasing number of molecules. The coordination sphere of Li + saturates with five acetonitrile molecules, which reflects in increased distances and in a markedly decreased interaction energy for the cluster containing five acetonitrile molecules. The other two ions can easily accommodate up to six acetonitrile molecules. The interaction is essentially electrostatic, but ligand polarization contributions are significant.

Comparative conformational analysis of n‐paracyclophanes by molecular mechanics

A molecular mechanical method developed by Boyd et al. has been used to calculate optimal geometries, vibrational frequencies, gas phase thermodynamic functions, heats of formation, and strain energies for the n‐paracyclophanes for 5≤n≤10. The results for the geometry and strain energies have been compared with those obtained by Allinger et al. for the same compounds, both methods being found to yield similar degrees of agreement with available experimental data.

A COMPARISON OF TWO METHODS FOR DIRECT TUNNELING DYNAMICS : HYDROGEN EXCHANGE IN THE GLYCOLATE ANION AS A TEST CASE

Two methods for studying tunneling dynamics are compared, namely the instanton model and the approach of Truhlar and co-workers, which are based on the direct output of electronic structure calculations and thus are parameter free. They are employed to evaluate the zero-level tunneling splitting due to intramolecular hydrogen exchange in the glycolate anion. The first method was developed in a series of recent studies and presents a combination of the instanton theory with quantum-chemically computed potentials and force fields. For the compound at hand, which has 21 internal degrees of freedom, a complete potential-energy surface is generated in terms of the normal modes of the transition-state configuration. It is made up of the potential-energy curve along the tunneling coordinate and harmonic force fields at the stationary points. The level of theory used is HF/6–31++G**. All modes that are displaced between the equilibrium configuration and the transition state are linearly coupled to the tunneling m...

The study of A(CH3OH)1–6 (A = Li+, Na+) in the gas phase based on ab initio calculations, analysis of the solvation process

Abstract A theoretical study of complexes formed by clusters consisting of up to six methanol molecules and Li + or Na + ions was performed with the HF, MP2, and DFT/B3LYP methods using the 6-31G* basis set. Several stationary points for each cluster were thus located and were characterized as minima from the frequency calculations. The intermolecular interaction energies of the cluster minima, corrected for the basis set superposition error, were obtained. Additional properties, such as the enthalpy and Gibbs free energy changes, and the solvent–solvent interaction energy, were also calculated. From the results, it follows that both Li + and Na + ions form similar complexes in the clusters with up to four methanol molecules; in the Li + complexes, however, the first coordination shell saturates with four molecules and leads to conformations differing from those of the Na + complexes. All the properties studied change in a systematic manner upon addition of a further methanol molecule; this trend, however, is broken by the Na + complexes with five and six methanol molecules owing to the formation of hydrogen bonds between methanol molecules. Li + -containing complexes exhibit no sign of hydrogen bonding between the molecules of the first coordination shell. Overall, the theoretical results are acceptably consistent with their experimental counterparts and allow one to identify some features of the clustering process.

Ab initio-gradient optimized molecular geometry and conformational analysis of 1,3-propanediol at the 4-21G level

Abstract The geometries of twenty five conformations of 1,3-propanediol were refined by the ab initio gradient method at the 4-21G level. The two most stable conformers are found to be stabilized by internal hydrogen bonding. Several conformationally dependent structural trends found previously in similar systems have been confirmed. The results are consistent with experimental data.

Intramolecular proton transfer in 2-(2′-hydroxyphenyl)benzoxazole: the reliability of ab initio calculations on simplified structures

Abstract It is common practice in Computational Chemistry to model the behaviour of bulky compounds from simplified structures. This procedure enables the use of higher computational levels, with generally improved results. The most crucial problem in this methodology is choosing how simple the structures in question should be, in fact, oversimplification in this context can lead to a highly different behaviour relative to the starting one. In order to check how valid this approach is, in this work we modelled intramolecular proton transfer in 2-(2′-hydroxyphenyl)benzoxazole in its ground and first excited singlet states from structures simplified to a variable extent. Ab initio calculations at the HF/3-21G level in the ground state and the CIS/3-21G level in the excited state included geometric optimization of potential tautomers and the intervening transition states. The results obtained reveal that the structure of the original compound must be simplified carefully if spurious conclusions are to be avoided.

A new intermolecular polarizable potential for cis-formic acid. Introduction of many-body interactions in condensed phases

Abstract An HCOOH–HCOOH intermolecular potential that includes many-body effects intended for computations of large clusters of formic acid and simulations in the liquid phase was constructed from monomer properties and ab initio calculations. The potential accurately predicts the minimum energies of the dimer. Predictions are also given and compared with ab initio calculations for formic acid trimer and tetramer. The results suggest that the lowest-energy structure in large clusters consists of a cycle formed via hydrogen bonds that folds as two parallel chains linked at one end. Simulated results suggest a liquid phase consisting of small molecular chains. Most likely, omission of induction effects will to results in a less structured liquid.

Analysis of the effect of substitution on the intramolecular hydrogen bond of malonaldehyde by ab initio calculations at the 3–21G level

Abstract We carried out an ab initio study of various derivatives of malonaldehyde by using the 3–21G basis and full optimization in order to analyse the influence of substituents on the features of the intramolecular hydrogen bond. The results show the hydrogen bond to be strengthened by electron-releasing and weakened by electron-withdrawing substituents. The estimated hydrogen-bond energies of all the compounds studied are closely related to the corresponding O…O bond distances. We also studied the intramolecular proton transfer by tunnelling and found its rate to be lower than that in the parent compound (malonaldehyde ). This lowering is attributed to the fact that the substituents raise the energy barrier.